1. Field of the Invention
The present invention relates to a toner for developing static charge images in electrophotography, electrostatic recording and electrostatic printing, and a developer, a toner container, a process cartridge, an image forming apparatus and an image forming method using the toner respectively.
2. Description of the Related Art
The image forming method for electrophotography, electrostatic recording and electrostatic printing, etc. includes development step in which a toner contained in a developer is attached once to an image bearing member such as photoconductor on which a static charge image is formed, transferring step in which the toner is transferred from the photoconductor to a transfer medium such as transfer paper and fixing step in which the toner is fixed on the paper.
In the fixing step, the surface of the roller has been formed of a material such as silicone rubber or fluorine resin having an excellent releasing property relative to the toner in order to prevent attachment of the toner onto the surface of the fixing roller and a thin film of liquid having a high releasing property such as silicone oil and fluorine oil is applied to the roller surface in order to prevent offset and fatigue of the roller surface.
This method is significantly effective in terms of preventing offset of the toner, however, since a feeding unit for offset preventing liquid is needed making the fixing apparatus more complicated, it is disadvantageous for energy conservation and moreover, separation between layers making up the fixing roller is induced by the oil application leading to facilitation of short life span of the fixing roller.
For this reason, oilless fixing apparatuses which do not employ feeding units for silicone oils have been proposed recently.
Because the toner used for this oilless fixing is needed to have releasing property relative to the surface of the fixing member to some extent, the viscoelasticity of the toner is increased by increasing polymerization degree of the resin, or instead of applying oil on the surface of the fixing roller, releasing agents such as low-molecular-weight polypropylene is added in the toner particles to feed the offset preventing liquid from the toner particles during heating and to provide peel property relative to the surface of fixing member.
For example, an oilless color toner in which releasing agent-including resin particles, which are granulated by mixing releasing agent emulsion in an emulsified polyester dispersion liquid and colored with dyes, is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 7-56390. By having such composition, a toner which excels in color reproducibility, and exhibits excellent offset resistance, winding resistance and fixing property even in oilless fixing can be obtained.
At the same time, toners used in these thermal-roller type fixing apparatuses are desirably having a lower limit of fixing temperature lowered as much as possible while maintaining the hot offset resistance.
In particular, many oilless fixing apparatuses such as above are often equipped with cleaning rollers which are in contact with fixing rollers or pressure rollers for removal of the toner attached to the surface of the fixing roller. When such fixing apparatuses are used for prolonged periods, the toner accumulated on the fixing cleaning roller is melted by heat and causes inverted hot offset, a defect caused by the melted toner which is reversely transferred to the fixing rollers or pressure rollers.
For this reason, approaches have been made to prevent melting of the toner from the cleaning rollers by decreasing the working temperature of the fixing rollers and fixing cleaning rollers.
When the thermal-roller type fixing apparatuses are used with the decreased fixing roller temperature, more improvement on low-temperature fixing property of the toner is required.
However, decreasing the fixing temperature of the toner poses a problem of difficulty in securing fixing temperature regions (hot offset resistance) and maintaining heat-resistant storage property.
As a method to satisfy both demands, making the molecular weight distribution of toner binders a wide region including low molecular weight to high molecular weight, have been proposed in the past (Japanese Patent Application Publication (JP-B) Nos. 60-20411 and 51-23354, for example).
However, when molecular weight of the toner binder is reduced in the low molecular weight regions or fixing temperature of the toner is lowered by increasing the ratio of low molecular weight components, storage stability in high-temperature regions is degraded and fusion during running becomes notable and also, troubles such as deterioration of image quality due to the change in charged amount may likely to occur.
Moreover, toners which excel in low-temperature fixing property, hot offset resistance and heat-resistant storage property, which are obtained from manufacturing methods including molecular-weight increasing step in which polyaddition reaction of isocyanate group-contained polyester prepolymer with amine in organic solvents and aqueous media is performed have been disclosed (JP-A Nos. 2002-287400 and 2002-351143, for example).
However, when the toner, which is produced by the above methods, is used with the decreased fixing roller temperature, fixing becomes insufficient and more improvement of low-temperature fixing property is required.
Furthermore, in the electrophotographic image forming in general, an electrical latent image is formed on a photoconductor, which is prepared by using photoconductive material, by means of various units. After the latent image is developed using a developer, the developed image is transferred to paper, etc. accordingly and then fixed by heat, pressure or solvent moisture.
The development methods of electrical latent images can be classified broadly into two categories: liquid developing method in which liquid developers prepared by finely dispersing various pigments or dyes in insulating organic liquids is employed, and dry developing method in which dry developers (hereinafter may be referred to as “toner”) prepared by dispersing colorants such as carbon black in resins as in cascade development, magnetic brush development and powder cloud development. Of these, dry developing method is widely used in late years.
The heat roller is widely used in general for the fixing in the dry developing method because of its excellent energy efficiency. Furthermore, thermal energy provided for the toner during fixing is tend to be reduced in an attempt to enhance energy conservation by the decrease in fixing temperature of the toner in recent years. A technology procurement project for copiers of next generation exists in the DSM (demand-side management) program of International Energy Association (IEA) in 1999 and its requirement specifications have been officially announced. For the copiers of 30 cpm or more, achievement of dramatic energy conservation compared to traditional copiers such as within 10 seconds of waiting time and 10 watts to 30 watts or less (depending on copying speeds) of power consumption during waiting, is required. As one of the methods to fulfill the requirements, temperature-responsive property of the toner may be improved by decreasing the heat capacity of the fixing member such as heat rollers; however, it is not satisfactory.
In order to fulfill the above requirements and significantly shorten the waiting time, lowering the fixing temperature of the toner itself in order to lower the fixing temperature of the toner when usable is considered to be technically an essential fulfillment items.
In an attempt to meet such decrease in fixing temperatures, polyester resins having excellent low-temperature fixing properties and relatively favorable heat-resistant storage properties are being tried for use in place of frequently used styrene-acrylic resins (JP-A Nos. 60-90344, 64-15755 2-82267 JP 3-229264, 3-41470 and 11-305486). Moreover, an attempt to add specific non-olefin crystalline polymers in binders for the purpose of improving low-temperature fixing property (JP-A No. 62-63940) and an attempt to employ crystalline polyesters (Japanese Patent (JP-B) No. 2931899) have been proposed, however, molecular structure and molecular weight of the polyester resins are not optimized in these proposals.
Furthermore, it is impossible to fulfill the specifications of the DSM program even if these known conventional arts are applied, and the establishment of low-temperature fixing technology which is more advanced than conventional technologies is needed.
For further decrease in fixing temperatures, controlling heat properties of the resin itself becomes necessary, however, if the glass transition temperature (Tg) is lowered too much, heat-resistant storage property may be degraded and if the molecular weight is reduced and the F1/2 temperature of the resin is lowered too much, hot offset generation temperature may be lowered. Because of these issues, a toner having an excellent low-temperature property and high hot offset generation temperature have not yet been obtained by controlling heat properties of the resin itself.
Next, manufacturing method of the toner used for developing static charge images can be broadly classified into pulverization and polymerization.
In pulverization, colorants, charge controlling agents and offset preventing agents are fusion mixed and dispersed evenly in a thermoplastic resin and a toner is produced by pulverizing and classifying the obtained toner composition. It is possible to produce the toner which has excellent properties to some extent by pulverization; however, material selection is limited. In other words, the toner composition obtained from fusion mixing has to be capable of being pulverized and classified by means of an affordable apparatus. Because of this requirement, fusion mixed toner composition must be sufficiently brittle. When the toner composition is actually pulverized to become particles, the mass average particle diameter of the toner must be reduced, for example, in order to obtain copied images with which particle diameter distribution of broader region is likely to be formed with appropriate resolution and tone, and there is a disadvantage of having extremely low toner yield because fine powder of 4 μm or less particle diameter and coarse powder of 15 μm or more particle diameter must be removed by classification. Moreover, it is difficult to disperse colorants or charge controlling agents evenly in a thermoplastic resin in pulverization and uneven dispersion brings harmful effects on flowability, developing property, durability and image quality of the toner.
In late years, manufacturing methods of toner using polymerization have been proposed and operated in order to overcome these problems associated with pulverization. For example, toner particles are obtained by suspension polymerization or emulsion polymerization condensation (JP-B No. 2537503).
However, it is difficult to produce the toner by using polyester resins which are advantageous in low-temperature fixing properties in these manufacturing methods of the toner.
To settle above issues, a toner of polyester resin which is spheronized in water using solvents (JP-A No. 9-34167) and a toner using isocyanate reaction (JP-A No. 11-149180) have been proposed, for example. However, low-temperature fixing properties and toner productivity were not sufficient in any of these proposals.
Therefore, the toner which is capable of pursuing excellent low-temperature fixing property and offset resistance simultaneously to form appropriate images of high resolution and related techniques thereof are not yet provided and their prompt provision is desired in the present situation.